During last funding period, we identified and established that TWEAK cytokine is a major regulator of skeletal muscle mass during aging and disuse conditions. We have now obtained strong evidence suggesting that the TWEAK causes insulin resistance in skeletal muscle. However, the mechanisms by which TWEAK induces muscle atrophy or insulin resistance remain less well understood. PGC-1? cofactor plays a major role in regulation of skeletal muscle mass, fiber-type composition, and mitochondrial biogenesis. Our preliminary studies have shown that TWEAK suppresses the expression of PGC-1? and mitochondrial content and inhibits insulin signaling in skeletal muscle. We have also found that disuse conditions or type II diabetes involve the inducible expression of TWEAK receptor Fn14 (but not TWEAK itself) in skeletal muscle. Our initial analysis has revealed that both human and mouse Fn14 promoter contains CpG rich regions and that demethylation of Fn14 promoter could be a crucial event for the induction of Fn14 expression in skeletal muscle. In the next phase of this project, we will investigate the mechanisms by which TWEAK causes sarcopenia, disuse-related muscle atrophy, and insulin resistance, and how the expression of Fn14 gets up regulated in skeletal muscle in different conditions. We will address the following three specific aims: AIM I. Investigate the role of PGC-1? in TWEAK-mediated skeletal muscle atrophy in vivo. Hypothesis 1: TWEAK causes muscle atrophy and fiber-type switching through down-regulation of PGC-1? expression. AIM II. Investigate the key mechanisms by which TWEAK-Fn14 system induces insulin-resistance in adult and/or aged skeletal muscle. Hypothesis 2: TWEAK induces insulin resistance by inhibiting insulin receptor substrate (IRS1) phosphorylation and diminishing oxidative phosphorylation in skeletal muscle. AIM III: Investigate the molecular events leading to increased expression of TWEAK receptor Fn14 in skeletal muscle. Hypothesis 3: Demethylation of CpG islands in Fn14 promoter followed by binding of SP1 transcription factor triggers the expression of Fn14 in skeletal muscle in catabolic states. Successful completion of this project will provide critical insights about sarcopenia durin aging and type II diabetes and provide novel avenues for therapeutic interventions.